Caribou (Rangifer tarandus) are wild reindeer in North America. They are an Arctic and Subarctic-dwelling deer, widespread and numerous across the north. Their weight varies between 130-370lbs (60-170kg) for females, while males can weigh up to 660lbs (300kg). Both sexes grow antlers. Reindeer hooves adapt to the season: in the summer, when the tundra is soft and wet, the footpads become spongy and provide extra traction. In the winter, the pads shrink and tighten, exposing the rim of the hoof which cuts into the ice and crusted snow to keep the animal from slipping. This also enables them to dig down through the snow to their favorite food, a lichen known as reindeer moss. Aside from lichen, they also eat the leaves of willows and birches, as well as sedges and grasses. The reindeer travels the furthest of any terrestrial mammal. The caribou of North America can run at speeds up to 50 mph (80kph) and can travel as much as 3,100 miles (5,000km) a year.